This invention relates in general to superconducting accelerator magnets and, more specifically to a method and apparatus for operating gas cooled leads for charging superconducting coils and the like at high voltage potentials.
Magnetic fields guide particles, such as protons, through beam tubes. Particles can be accelerated to speeds approaching the speed of light by accelerators made up of a number of axially arranged high field magnets, with beam tubes under high vacuum that contain the particles.
In high energy physics research, such magnets have been used to accelerate and guide particles and cause collisions between them to reveal the presence of more fundamental particles and forces. Particle accelerators are also used in medical research and treatment, where tissues are bombarded with selected particles to change or destroy selected types of tissue, such as tumors. Other applications include x-ray lithography and protein crystallography
Superconductors are materials, typically metals or ceramics, that lose all resistance when cooled below a critical temperature. Many materials have superconducting capabilities, although most only superconduct at temperatures approaching 0.degree. K. The most practical superconductors for use in superconducting magnets are those that superconduct at or above the boiling temperature of liquid helium. Nb--Ti and Nb.sub.3 Sn are the most common superconducting materials. Recently, ceramic superconductors, such as YBa.sub.2 Cu.sub.3 O.sub.7 have been developed that have critical temperatures above the boiling temperature of liquid nitrogen.
Magnets formed from superconductors and cooled below their critical temperatures are highly efficient and can provide extremely high magnetic fields. Such magnets are used in particle accelerators used in medical treatment, physics research, superconducting magnetic energy storage and other fields. The Superconducting Supercollider will use thousands of superconducting magnets to guide particles through a very long, multi-magnet tube.
Because of the extremely low temperatures at which the magnets operate, a complex very efficient thermal insulation system must be provided. Typically, the magnet coils may be cooled by liquid helium in a vessel surrounding the coils. A vacuum vessel surrounds the helium vessel, surrounded in turn by a liquid nitrogen shield and high efficiency multilayer film insulation.
Superconducting magnets require leads penetrating through this insulation system in order to charge and discharge the magnet coils as necessary. These leads are a source of very significant heat leaks into the system, which can cause excessive boil-off of the liquid helium and liquid nitrogen. In order to reduce this heat leak, the leads are conventionally cooled by boiling liquid helium vapor.
The vapor cooled leads must be at high voltage potentials to ground during operation of the magnet coils while at the same time the gas is being recovered by equipment at ground potential. Due to the poor insulating qualities of helium gas, the leads have typically been limited to about 500 volts to ground. With large magnet systems, the ability to use a much higher potential at these leads would permit more rapid magnet charging and discharging and over-all much more efficient operation of the magnet system.
Thus, there is a continuing need for methods and apparatus that permit operation of electrical leads and the like that penetrate through superconductor insulation at much higher potentials.